This invention relates to the modular processing of stranded gases with methane components into a natural gas stream suitable for injection into pipelines in the United States and elsewhere without the need for blending with additional pipeline gas or utilizing cryogenic techniques.
In the last two decades nine projects were designed and constructed to process LFG and insert the product gas into natural gas pipelines. Many of these projects failed to provide a continuous acceptable product gas. Some, such as the Dominion Airtech project at the Alliance Landfill near Scranton, Pa., relied on controlling air intrusion at the landfill. None of the projects were able to address conditions where significant nitrogen or oxygen was present in the gas.
Many stranded gas assets exist in the United States. Landfills are a prime example of stranded gas as it is of relatively low quality and flow rate. Many oil wells concurrently produce natural gas with considerable concentrations of contaminants and the gas is very often flared at the site. Industry and government evaluations suggest that at least 25% of the United States natural gas supply contains unacceptable levels of contaminants such as nitrogen, hydrogen sulfide, or other compounds. Carbon dioxide is also a common contaminant and there are several commercial processes that can adequately separate this from methane, the most common of which is amine wash.
LFG, as produced by the anaerobic digestion of municipal organic wastes, is nominally a 55/45 mixture of methane and carbon dioxide, with trace contaminants of siloxane compounds, hydrogen sulfide and a number of volatile organic compounds (VOC's). The LFG collection is aided by the operation of blowers that create a negative pressure in the landfill. This negative pressure causes air infiltration into the LFG stream, especially at the periphery of the landfill where gas wells exist primarily to reduce the migration of LFG (and its corresponding odors) to nearby properties. This air infiltration introduces nitrogen and oxygen into the LFG. This contaminated gas stream has yet to be processed successfully. In the United States, the only commercially successful projects to process LFG rely either on utilizing only select wells in the landfill where the air content is very low, or blending of a lightly processed LFG stream with large quantities of pipeline gas, sometimes as high as twenty five parts of pipeline gas to one part LFG.
In some instances, LFG has been processed and accepted into pipelines at qualities less than normally required. Unless flows in the pipeline accepting lower quality processed gas render its contaminant contribution insignificant, this lower quality gas can cause difficulties for natural gas end users. Improper or less than optimal treatment of the stranded gas may result in a carryover of landfill gas contaminants into the pipeline and eventually into businesses or residences. Complete treatment of landfill and other non-standard gases will provide additional indigenous and sometimes renewable resources for the well-developed natural gas distribution systems in the United States. Landfill sites also have a major advantage in that they are often located near larger metropolises and corresponding high gas usage areas. Recovery of the uncontaminated methane for use in normal natural gas markets will result in a more efficient use of the energy content than the more usual use of LFG for electrical generation, and its attendant energy conversion losses.
The inventor is unaware of any systems in operation processing landfill or other stranded gases that remove substantially all contaminants, including hydrogen sulfide, non-methane organic compounds (NMOC's), carbon dioxide and air components (nitrogen and oxygen) without some form of cryogenic separation or partial cryogenic processing. Cryogenic processes are very capital intensive and use significant amounts of energy during production. They are not economic options for modest or smaller gas flows as normally encountered at landfills and other stranded gas production sites. Other types of systems employed on LFG projects including high pressure membranes, solvent based partial cryogenic systems such as Selexol® or Kryosol®, or most pressure (or temperature) swing absorption systems are not suitable for gas streams with air components and in most cases rely on significantly higher pressure requirements.